Reversible semiconductor and method of making it



W. G. PFANN Jan. 26, 1954 REVERSIBLE SEMICONDUCTOR AND METHOD OF MAKING I Filed Dec. 31, 1948 FIG. 2

lNl ENTOR W G. PE4NN ATTORNEY Patented Jan. 26, 1954 REVERSIBLE SEMICONDUCTOR AND METHOD OF MAKING IT William G. Pfann, Chatham,

N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 31, 1948, Serial No. 68,596

6 Claims.

This invention relates to translating devices and more particularly to translating devices employing a body of semiconductive material, and to methods of making such devices.

As is pointed out more fully in Patent 2,524,035, granted October 3, 1950, to J. Bardeen and W. H. Brattain, a body of semiconductive material provided with three suitable connections is capable of acting as an amplifier of electric signals. In one exemplary embodiment such an amplifier comprises a block or slab of N-type germanium material, means for making two closely spaced point contacts to one face of the block and means for making a large area contact to the opposite face. In such a device, the input circuit is between one point contact, called the emitter, and the large area contact, called the base, and the output circuit is between the other point contact, called the collector, and the base. For the N-type material indicated, with a small bias, usually positive, of a fraction of a volt on the emitter and a relatively large negative bias, of the order of tens of volts on the collector, small signal variations at the input appear as large variations in the output circuit with a resultant power gain.

As set forth in the applications of W. H. Brattain, Serial No. 67,781, filed December 29, 1948, and W. G. Pfann, Serial No. 67,797, filed December 29, 1948, certain forming and other treatments enhance the amplifying ability of devices of the type described, e. g. by improving the action at the collector connection.

One general object of this invention is to enhance the operating capabilities of semiconductor translating devices and especially of such devices operable as amplifiers.

More specifically, one object of this invention is to enable operation of a three contact translating device of the general construction above described in either direction, that is with either of the point contacts as the emitter and the other as a collector, without substantial variation in the operating characteristics, such as the gain.

A feature of this invention involves a semiconductor triode having two spaced restricted area or point contact connections on the surface of a semiconductor each of said connections being capable of performing either as an emitter connection or as a collector connection.

A further feature of this invention pertains to forming each restricted area connection in a device in accordance with the foregoing feature so that it will, when properly connected, serve as a collector of a semiconductor amplifier having substantially equal gain in either direction.

Other and further objects and features of this invention will appear more fully and clearly from the following description of an illustrative embodiment thereof taken in connection with the appended drawings in which:

Fig. 1 illustrates one form of translating device illustrative of this invention;

Fig. 2 shows an electrical circuit suitable for measuring the reversible amplifying properties of a translating device made in accordance with this invention, and

Fig. 3 is a circuit diagram of apparatus for forming the point contacts.

Semiconductors suitable for use in devices of this invention include germanium and like materials containing minute quantities of significant impurities which comprise one way of determining the conductivity type (either N- or P-type) of the semiconductive material. The conductivity type may also be determined by energy relations within the semiconductor. For a more detailed explanation, reference is made to Patent 2,524,035, which issued October 3, 1950, to J. Bardeen and W. H. Brattain.

The terms N-type and P-type are applied to semiconductive materials which tend to pass current easily when the material is respectively negative or positive with respect to a conductive connection thereto and with difiiculty when the reverse is true, and which also have consistent Hall and thermoelectric effects.

The expression significant impurities is here used to denote those impurities which affect the electrical characteristics of the material such as its resistivity, photosensitivity, rectification and the like, as distinguished from other impurities which have no apparent effect on these characteristics. The term impurity is intended to include intentionally added constituents as well as any which may be included in the basic material as found in nature or as commercially available.

In Fig. 1, only those parts of the translating device requisite to a description of this invention are shown, the housing and the means for supporting the elements in the housing having been omitted in the interest of clarity. At I i is shown a block or slab of semiconductive material, for example N-type germanium material. On one surface l2 of this block are located two restricted area connections or contact points I4 and which may be of Phosphor bronze or other suitable material. The junctions between surface I2 and the contacts I4 and I5 respectively are designated as I6 and I7 and are on the order of several thousandths of an inch apart. A large area connection 13 is made to the opposite face of block II by means of copper plating, solder or the like. Connection may be made to connection 13 by a conductor IS.

The N-type germanium material employed for the block ll may be prepared in the manner described in the application .of J I-I. Scaff and H. C. Theuerer, Serial No. 638,351, filed December 29, 1945, or as described in Crystal Rectifiers by Torrey and Whitmer, vol. 15, Radiation Laboratory Series (MIT).

Electrical forming procedures for .improving the amplification of a translating device of this type are described in the applications of W. H. Brattain and W. G. Pfann, previously noted. Such a method comprises passing through one of the point contacts, known as the collector, sufficient current to exceed the voltage maximum in the reverse direction through this connection with the other contact, calledrthe emitter, connected in the circuit. As a result of such a forming operation, the device becomes a unilateral amplifier. That is, the connection which is formed acts as a collector and is subsequently used as a collector for best-results. If the other, unformed point contact is made the collector, very low power gain or possibly power loss will result. a

The forming may be done in one way by means of suitable apparatus connected in a circuit such as is shown in Fig. 3. This circuit may also be used to examine the amplifying properties of the semiconductor translator. The connection M which is to be the emitter is connected to one terminal of a low impedance secondary 4| of a" transformer across the primary of which is impressed an alternating voltage of about 1,000 cycles per second from a signal source 42. The other terminal of winding 4| is connected to a tap 33 of a potentiometer M across which is connected a source of voltage such as battery 45 poled as indicated in Fig. 3. The negative terminal of 45 is connectedto the base connection it. The voltage of source 45 maybe from'fi to'lZ volts and the resistance of M may be 500 ohms or less. The tap is ordinarily adjusted so that the effect of one volt in series with 200 ohms or less is achieved. The amplitude of the signal across winding Ql may be about 0.1 volt.

One end of a low impedance secondary winding 58 of an isolating transformer is connected to. the connection l5 of the translating device which is to be the collector. An adjustable source '49 of 60 cycles per second alternating voltage is connected across the primary of the isolating transformer. The voltage available across winding-48 should be from to about 135 volts. The other end of the winding as is connected through .a resistor as to the base connection 13. The resistor 50 should be on the order of 5,000 ohms or larger.

To observe the relationship between the current through the collector l and the potential difference between the collector I5 and the base I3 suitable connections are made to an indicator such as the cathode ray oscilloscope 60. The voltage across the resistor 50 is applied to the plates 6i and 62 and that between the collector l5 and the base connection 13 to the plates 53 and 64.

With one volt or less about volts across winding 48 the oscilloscope will show a reverse characteristicpf the collector .15 withthe 1,000-cycles per-second-signal vfrom source 42 superposed thereon. ii -;the voltage on the emitter l4 and .show that a translating emitter and the point shown in Fig. l, a reversible amplifier,

across 48 is then increased until the reverse characteristic of the collector is in the negative resistance region forming will result. This will be indicated by a considerable increase in the amplitude of the signal as seen on the oscilloscope E0. The forming may be controlled by varying both the voltage between I3 and I4 and between l3 and i5. Care must be taken not to overform by use of too high a voltage since this will result in an unstable condition with little or no improvementin gain of the amplifier.

The circuit illustrated in Fig. 2 may be used to device formed in the manner above indicated will act as a unilateral In this figure the elements of the device are indicated as in Fig. l, i. e., :block .H, thecontact points l4 and I5, and the base connection l3. The base connection l3 on the one hand is connected via conductor is through a resistor 22(R1), a biasing source, such as battery 20 and a signal source 2! to a reversing translating {switch 19. On the other hand, the base connection It is connected via conductor l8 through a biasing source, such as battery 23, a resistor 24(R2) to the reversing switch IS. The elements on the left side of the diagram are those ordinarily associated with an emitter and those on the right of the diagram those ordinarily associated with a collector. The reversing switch l9 hastwo positions A and B such that in the position A the contact !5 is connected as the emitter and the contact l 4- as the collector and in position 13 the contact 14 is the emitter and the contact l-fi-the collector. If N-type germanium material is used for the block H the biasing source 20 is of the order of one volt or less and the biasing source-23 of the order of a few volts to 50 or more volts. In such a circuit the resistor 22(R1) may be of the order of 200 ohms and the resistor 24(R2) of the order of 10,000 ohms.

The unilateral translating device tested in the circuit of Fig. 2 with the reversing switch H! in position 3, that is, with the point l4 as the IS as the collector, will show a power gain of the order of (20 decibels). The power gain in this case may be defined as follows:

where V1, V2 and'Vs are respectively the alternating current voltages across source 2 I, between connections l3 and i l and across resistor 24(Rz). On the other hand, when the reversing switch is in position A with the electrode It as the col- :lectorand'l'fi as the emitter, the values of voltages V1, V2 and V3 will be such that a relatively low powergainor possibly a power loss is indicated.

If it is desired to make a device such as that which willainplify the same in either direction, it may .bedone byforming each of the contacts l4 and J5 as a collector. This may be done for example by forming first one contact, say [5, as the collector in accordance with the procedure before 'noted and then interchanging the connections and forming the other contact, say 14, asa collector. Although both of the point contacts are now formed to act as collectors in relatively high gain amplifiers, this forming appears to in no way seriously degrade the operation of either as an emitterwhen so connected.

After these forming stepsfor each of the connections the amplification ,in each direction at given values of emitter and collector biases, may

not be exactly the same in some cases. When this is true and equalization is desired, it can be approached by passing a steady or pulsating forward current through the point having the while biasing a second restricted area connection with a forward voltage relative to the base.

4. The method of making a reversible semiconductor amplifier having spaced rectifying contacts and. an ohmic base connection on a greatest amplification. The emitter circuit is left open during this step. In the case of N- body of N-type germanium material that comtype semiconductor material for the block H the prises forming each of said contacts as a colforward direction is that for which the collector is l t r y app y t t an a te nati voltbiased positively with respect to the base. Steady age from a 011r0e 0f the Order Of 100 Volts currents on the order of 0.1 to 1.0 ampere have thrPugh a reslstance of Order of 5 9 ohms been found effective in reducing the ammmcw while the other contact is for each forming subtion of the device in the desired direction. Jegted a i g ti loltagle1 from a source (if $11 or er 0 one vo roug a resis ance o .e Another f q which may be used to equatorder of not more than 200 ohms. we the amplification is to reform by the previ- 5 Th method of m ki I noted over-the-oeak method the electrode 15 e a a revelsl e ous y conductor amplifier having spaced rectifying for which the amphficatlon 15 the lower when contacts and an ohmic base connection on a body used as a 30118017013 of N-type germanium material that comprises Referring to 2 for nomenclature, the forming each of said contacts as a collector by spective gains of a reversible unit in each disuccessively applying thereto an alternating voltrection, measured in accordance with the variage from a, o r of th order of 100 volts ous indications on Fig. 2 are as follows: through a resistance of the order of 5,000 ohms Switch Position V1 V2 Va R1 R2 u Io En In Gain A .088 v. .038 V. 5.2 V. 200w 10,000w -20 V. 2.41113. -|-.03 v. .48 ma. 24.5 db B .088 v. .038 v. 4.7 V. 2000: 10,0000: 20 v. 2.8 H18. +.03 v. .55 ma. 23.7 db

As indicated by the above figures, the device while the other contact is for each forming subis capable of giving substantial gain in either dijected to a forward voltage from a source of the rection and this gain is about the same in each order of one volt through a resistance of the case. order of not more than 200 ohms to produce Although this invention has been described substantially equal amplification in each direcwith respect to a particular embodiment thereof tion through the amplifier, and then equalizing it will be understood that various changes may be any small differences of amplification by passing made therein without departing from the spirit a forward current of the order of 0.5 ampere and scope of the invention. through the rectifier contact responsible for the What is claimed is: higher gain when it is a collector.

1. The method of making a reversible semi- 6. The method of making a reversible semiconductor amplifier having two spaced restricted conductor amplifier having spaced rectifying area connections and a large area base conneccontacts and an ohmic base connection on a body tion on a semiconductive body, that comprises of N-type germanium material that comprises electrically forming one restricted area ccnnecforming each of said contacts as a collector by tion by passing a relatively high reverse current pplying thereto an alternating Voltage from a therethrough while both restricted area connecsource of the order of 100 Volts through a resisttions are in circuit with the base connection to ance of the order of 5,000 ohms while the other condition said one connection as a collector con- Contact is for each forming j ct d to a folnection and then likewise electrically forming W d oltage from a source of the order of one the other restricted area connection as a 001- Volt through a eSistance of the order of 200 lector connection ohms to produce substantially equal amplifica- 2' In the method of making a reversible semi tion in each direction through the amplifier, and conductor amplifier having restricted area conthen equalizing y Small d e c s o amplinections and a base connection in spaced relation fi qn y fo ming the rectifier contact reon a body of semiconductive material that com- Sponslble for the 10We1" a plification when it is prises forming each restricted area connection a collectorby passing an electrical current therethrough to WIDZLIAM G. PFANN.

roduce substantially equal amplification in ither direction through the amplifier, the step References 01M 1n the file of @1118 patent of further equalizing the amplification that com- UNITED STATES PATENTS prises reducing the forming effect on the restrict Number Name Date ed area connection responsible for the higher am- 1,831,744 Laurent Nov 10 1931 fillirizclagtrilon by passing a forward current there- 2,428,400 Van I'IIII; OCJD 1947 3. The method of making a reversible semi- 32333 gfi g, &2; igig conductor amplifier having restricted area con- 2524035 52 1950 nections and a base connection on a body of N- type germanium material, that comprises elec- OTHER REFERENCES trically forming each restricted area connection as a collector by passing a relatively high reverse "0 Engmeelmg, pgs. 28, 29, 39, August current therethrough to the base connection 

